Selective removal of chlorine from solutions of chlorine dioxide and chlorine

Abstract

Chlorine is selectively cathodically electrolytically removed from aqueous solutions of chlorine dioxide and chlorine by the application of an electrode potential at a pH of the aqueous solution of up to about 4. Chlorine is electrolytically reduced to chloride ions while the concentration of the chlorine dioxide remains substantially the same in the treated solution. The chlorine reduction is effected directly at pH values up to about 2 using a low overpotential cathode and indirectly at pH values up to about 4 using a high overpotential cathode.

What I claim is: 
     
       1. A method of treatment of an aqueous solution of chlorine dioxide and chlorine, which comprises selectively cathodically reducing at least a portion of said chlorine at a pH of said aqueous solution of up to about 4, without substantially decreasing the concentration of said chlorine dioxide in the resulting treated solution. 
     
     
       2. The method of claim 1 wherein said selective cathodic reduction of chlorine is effected directly at a pH of no greater than about 2 using an electrode having a low overpotential for the electrolytic reaction Cl 2  →Cl -   at an applied potential of about +0.8 to about +1.2 volts as compared with a saturated calomel electrode (SCE). 
     
     
       3. The method of claim 2 wherein said applied electrode potential is about 1 volt. 
     
     
       4. The method of claim 1 wherein said selective cathodic reduction is effected using an electrode which has an electroconductive surface constructed of an electroconductive metal, metal alloy, metal oxide or metal compound. 
     
     
       5. The method of claim 1 wherein said selective cathodic reduction of chlorine is effected indirectly using a high overpotential cathode at a pH of up to about 4 at an applied potential of about +0.8 to about -0.6 volts as compared with SCE, in accordance with the equations: ##STR2## 
     
     
       6. The method of claim 1 wherein said selective cathodic reduction is effected using a high surface area cathode having a three-dimensional, electrolyte-contacting surface. 
     
     
       7. The method of claim 6 wherein said cathode comprises stacked layers of electroconductive mesh material through the interstices of which percolates the aqueous solution of chlorine dioxide and chlorine generally parallel to the current flow. 
     
     
       8. The method of claim 6 wherein said cathode comprises a packed bed of individual electroconductive particles through which percolates the aqueous solution of chlorine dioxide and chlorine generally perpendicular to the current flow. 
     
     
       9. The method of claim 6 wherein said cathode is located in a cathode chamber of a cell, an anode is located in an anode chamber of the cell and the anode chamber and cathode chamber are separated by an ion-exchange membrane. 
     
     
       10. The method of claim 1 wherein said selective cathodic reduction is effected by applying a constant voltage to a cathode and an anode during the electrolytic reduction of the chlorine. 
     
     
       11. The method of claim 10 wherein the applied voltage between the anode and the cathode is less than about 2 volts. 
     
     
       12. The method of claim 1 wherein said aqueous solution of chlorine dioxide and chlorine contains about 0.1 to about 20 g/L of chlorine dioxide and about 0.01 to about 10 g/L of chlorine.